The invention relates to a torque control apparatus for a pneumatic impact wrench driven by an air motor. The apparatus according to the invention makes it possible to simplify the control mechanism of compression torque, to control it with precision, and to change the control valve with ease, by providing a fluidic element and a torsion bar in a pneumatic impact wrench.
A pneumatic impact wrench provided with such torque control apparatus has already been disclosed in the U.S. Pat. No. 3,948,328.
In the torque control apparatus according to the said prior art, however, a spring-system reducing valve was used intermediately in an air passage communicating with an inlet of the fluidic element. Such spring-system reducing valve had a disadvantage in that the air pressure supplied to the fluidic element was unstable due to vibrations of the valve body through the elasticity of the springs.
Moreover, the main valve for intercepting the supply of air to the air motor was adapted to recover also by the force of springs. Due to high resistance of the springs, it occasionally happened that the valve could not be actuated by a small outlet pressure of the fluidic element. The conventional apparatus, therefore, had a further disadvantage in that the operation of the main valve was unstable.